Telegraph transmitter control system



Oct. 17, 1961 SELLERS, JR 3,005,041

TELEGRAPH TRANSMITTER CONTROL SYSTEM Filed Dec. 31, 195'? 4 Sheets-Sheet1 lNl/ENTOR G. A. SELLERS JR.

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. ATTORNEY:

Oct. 17, 1961 G. A.'SELLERS, JR 3,

TELEGRAPH TRANSMITTER CONTROL SYSTEM Filed D60. 51, 1957 4 Sheets-Sheet2 wins/v70 6. A. SELLERSJR- 1 4 ATTO NEY FIG. 2

Oct. 17, 1961 e. A. SELLERS, JR 3,005,041

TELEGRAPH TRANSMITTER CONTROL SYSTEM INVENTO/P G A. SELLERS, JR. 8)

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Oct. 17, 1961 G. A. SELLERS, JR 3,005,041

TELEGRAPH TRANSMITTER CONTROL SYSTEM Filed Dec. 31, 1957 4 Sheets-Sheet4 INVENTOP G. A. SELLERZJR ATTORNEY FIG. 4

United States Patent 3,005,041 TELEGRAPH TRANSMITTER CONTROL SYSTEM GabeA. Sellers, In, Summit, N.J., assignor to Bell Telephone Laboratories,Incorporated, New York, N.Y., a corporation of New York Filed Dec. 31,1957, Ser. No. 706,462 11 Claims. (Cl. 178-41) This invention relates tosignal responsive selector mechanisms and particularly toinstrumentalities for making selections under the conjoint control oftwo or more telegraph code combinations received in succession.

More especially, the invention relates to an electronic circuitarrangement for decoding and giving effect combinationally to two ormore successively received telegraph signals.

An object of the invention is to distinguish by means of combinations ofcircuit components between intervals of difierent durations interveningtwo successively received telegraph signals.

Another object of the invention is to qualify selectively operablecircuits to respond to their selecting signals only upon the detection,by combinations of circuit components, of a predetermined intervalbetween two particular successively received telegraph signals.

Another object of the invention is to cause the automatic generation ofa permutation code signal combination by means of a circuit arrangementincluding electron discharge devices.

Another object of the invention is to generate marking and spacingsignal elements by the conjoint output of a plurality of electrondischarge devices.

Another object of the invention is to pulse in difierential manner thecontrol grids of a plurality of electron discharge devices having acommon output circuit to cause the generation in said output circuit ofmarking and spacing elements.

A feature of the invention is a circuit arrangement associated with aninterval timer for selectively disabling the timer before completion ofthe timing operation or precluding registration of completion of thetiming operation.

Another feature of the invention is a bistable trigger circuitassociated with a monostable interval timing trigger circuit andconditionable by said monostable trigger circuit for operation, andbistable trigger circuit, upon operation, precluding self-restoration ofsaid monostable trigger circuit, and upon restoration, forcingrestoration of said monostable trigger circuit.

Still another feature of the invention is the provision of a delaycircuit including a diode between the output circuit of a discharge tubeand the input circuit of another discharge tube for delaying response ofthe latter tube to a controlled change in conductivity in the formertube.

Yet another feature of the invention is a telegraph signal transmissioninstrumentality controllable from the common output circuits of aplurality of electron discharge devices to cause the transmission of onekind of signal element when all of the discharge devices are in onecondition of conductivity and to cause the transmission of another kindof signal element when at least one of said electron discharge devicesis in another condition of conductivity.

Patent 2,766,318, granted October 9, 1956 to W. M. Bacon, G. J. Knandel,J. A. Krecek, and G. A. Locke, discloses, as part of an automaticteletypcwn'ter switching system, a multistation line having one or morestation control circuits each arranged to connect a teletypewriterstation in message receiving relation to the line in response to addresscodes each comprising two ice permutation code signal combinations. Anelectromechanical selector mechanism for making a station selection inresponse to the combined selective attributes of two or more receivedcode combinations is disclosed generally in the Bacon et al. patent, andis disclosed in considerably greater detail in Patent 2,543,174, grantedFebruary 27, 1951 to G. G. Keyes and in Patent 2,568,- 264, grantedSeptember 18, 1951 to W. J. Zenner. This electromechanical selectormechanism includes a plurality of pairs of individually selectableelements, one element being normally blocked against selection by ablocking lever which the other element, upon selection, disables. Uponreception of a particular code combination all of the unblocked orprimarily selectable elements that are coded to respond to that codecombination are selected and moved to operated positions in which theybecome latched. In the latched positions, the primarily selectableelements unblock their associated secondarily selectable elements. Anyunblocked selectable element is then free to respond to its selectivecode combination, providing that code combination is next received, toclose contacts or perform some other operation by which eifect is givento the selection, such as the selection of a teletypewriter station.Following the selection of a secondarily selectable element, all of theoperated primarily selectable elements are unlatched and restored tounoperated condition. The same signal may serve as the first codecombination of certain address codes and as the second code combinationof other address codes, so that in the same receiving cycle in which asecondarily selectable element is selected and operated one or more ofthe primarily selectable elements may also be selected and operated.Each address code comprising two code combinations is followed by aLetters signal, the primary purpose of which is to place ateletypewn'ter printer in the lower case or unshift condition, but whichis also sometimes used as an idling signal to provide time for switchingor other operations or to perform supplementary functions notinconsistent with its use as an. unshift signal. The supplementaryfunction that the Letters signal performs as it follows the twocharacteraddress code is the unlatching of the primarily selectable elements thatoperate and latch in the same receiving cycle and in response to thesame code combination as a secondarily selectable element.

It is the purpose of the system disclosed herein to perform thefunctions of the electromechanical selector mechanism by an electricalcircuit arrangement of circuit components particularly employing spacedischange devices and semiconductor devices.

Briefly, the electrical system for making selections in response to twoor more code combinations successively received includes a normallydormant oscillator that is held dormant under the control of a telegraphreceiving relay associated with a telegraph line, the relay being heldin a steady marking condition by the current that flows in the rest.condition of the line. When the receiving relay responds to the startelement of a telegraph code combination, it sets in operation theoscillator and also triggers a monostable flip-flop circuit to itsofi-normal condition. The flip-flop circuit includes timing elementssuch that without external control it will return to its normalcondition in an interval slightly longer than that of a received codecombination. The frequency of the oscillator is such that each cycle hasa duration equal to that of an element of correctly timed telegraph codesignals. The flip-flop circuit, in the off-normal condition, is pulsedby the oscillator once in each cycle thereof, but the circuit constantsof the flip-flop circuit prevent it from returning to normal conditionuntil the oscillator circuit pulse associated with the last selectingcode element of a received code combination occurs, at which instant theoscillator forces the re- 3 turn of the flip-flop circuit to normalcondition, the oscillator being thereby stopped. The oscillator thussupplies the external control that restores the flip-flop circuitslightly ahead of the time at whichits own circuit components wouldrestore it.

Associated with the receiving relay is a pulsing circuit for separatingreceived marking and spacing signal elements and for pulsing, under thecontrol of the oscillator circuit, one or the other of two conductivepaths, one of which is pulsed for marking elements and the other forspacing elements. The conductive pathsthat are pulsed selectively inaccordance with the marking and spacing nature of received code elementsare connected to discharge transferring cathodes of gas-filled steppingtubes according to the arrangement of apparatus and principle ofoperation disclosed in M. A. Townsend Patent 2,606,- 309, grantedAugust'5, 1-952. This patent discloses a gas-filled stepping tube inwhich there is directional selectivity in the stepping of a dischargefrom cathode to cathode in a fan circuit arrangement, whereby any one ofa plurality of cathodes may be selected under the control of two typesof code elements which may be designated as marking and spacingelements. The Townsend patent discloses a tube having eight finalcathodes which is an arrangement that is feasible of manufacture. Inaccordance with the teaching of Townsend, a discharge may be transferredto any one of the eight final cathodes in three steps 'under to controlof three code elements. In the arrangement according to the presentinvention, four such tubes are employed to provide a total of thirty-twofinal cathodes, which is the maximum number of selective possibilitiesof the five-unit permutation code. Each of the tubes includes not threebut five steps, the first two of which are non-directional, the fanningbeginning at the third Step. By properly connecting the transfer controlcathodes to the marking and spacing pulsing leads, a discharge willappear at a difierent one of the thirty-two final cathodes for each codecombination of the five-unit permutation code.

The transfer of a discharge to a selected final cathode of one of thetubes causes a voltage to be impressed upon a memory or indicationcircuit connected to that particular final cathode. Each of the memoryor indication circuits is a bi-stable single transistor flip-flopcircuit having the arrangementand mode of operation described incopending application Serial No. 292,875, filed June 11, 1952 by B.Ostendorf, Jr., now Patent 2,831,983, issued April 22, 1958.

Each of the indication or memory circuits has associated with it one ormore combiner circuits, the number of those circuits associated with asingle memory circuit, depending upon the number of selections that havethe same first character. When a single transistor flipflop circuithasbeen operated to the off-normal condition in recognition of reception ofa character code, it causes the charging of a condenser in each of thecombiner circuits connected to it. At the beginning of the next codecombination received, the discharges are reset in the fan circuitgaseous stepping tubes, and upon the energization of. a final cathode inresponse to the second code combination, a pulse is applied to thecondenser of eachof the combiner circuits associated with the particularfinal cathode. In the case of a combiner circuit which has its condensercharged by the associated code storage circuit, the pulsing of thecondenser in the combiner circuit will cause the activation of a stationreceiving circuit. station receiving circuit is a two-condition circuit,and when activated, it renders an associated teletypewriter 'receivingprinter or reperforator responsiveto signals received by the receivingrelay. I

One or more of the station receiving circuits may be activated in themanner described above. When all of the address codes preceding amessage have been decoded in this manner and the appropriate stationreceiving circuits activated, the signals for carriage. return and linefeed follow and are received ahead of the text of the message for theselected receiving stations. In response to the decoded line feedsignal, an activate-deactivate circuit, which is a two-conditioncircuit, is operated to the deactivate condition, in which it imposesupon the station receiving circuits a condition preventing them frombeing activated in response to sequences of character codes that appearin the text of the message and are the same as address codes. At the endof the message, a disconnect signal comprising the codes for Figures,which is the upper case shift code, the opposite of the Letters signal,and an H signal operate through a memory circuit and associated combinercircuit to restore the activate-deactivate circuit to'the activatecondition, thereby rendering the station receiving circuits responsiveto their appropriate two-character address code signals.

' The present invention comprises an electronic selector circuitarrangement for controlling the starting of any one of a plurality oftelegraph transmitters asso ciated with a multistation line comprisingfull-duplex transmitting and receiving channels. A' roll call oftransmitters, seeking one that has been conditioned for transmission,comprises a succession of single character codes,

sion of the preparatory pattern of signals and the roll call, mayinterrupt message transmission for the purpose of making a roll call oftransmitters. It is desirable to prevent the recording of thetransmitter start roll call signals by the teletypewriter recorder ofany station or stations that have been rendered responsive to messagetransmission, and this is the reason for blinding them. When the rollcall has been completed, by the starting of a transmitter or bycanvassing all trans- ,mitters wthout finding one conditioned fortransmission. any teletypewriter recorder that was blinded is unblindedand message transmission is resumed;

The hereinbefore identified Bacon et al. patents disclose arrangementsfor starting selectively the transmitters at any of the stations on amultistation line under the control of transmitter start signalsreceived by an electromechanical selector mechanism from a switchingcenter to which the multistation line is connected. The types oftransmitter start signals employed in the two systems disclosed in theaforementioned Bacon et al. patents are somewhat different, but in thesystem shown in the Bacon et ,al. Patent 2,871,286, they consist of: aBlank signal, followed by a measured pause, during which the channelover which the signals are received remains lectively pertaining to thestarting of one of the transmitters; and, upon the starting of atransmitter which has been conditioned by the insertion therein of a message tape, or upon completion of a roll call of all transmitters withoutfinding any having a message awaiting transmission, a Letters signalindicating the end of a transmitter start'signal sequence. a

The present invention comprises an electronic selector circuitarrangement for responding to transmitter start signals of the typehereinbefore described and for controlling the starting of any one of aplurality of telegraph transmitters associated with a multistationline,- c'omprising full duplex transmitting and receiving chan-v nels.As disclosed in the Baconet al. Patent 2,871,286,. a transmitter startcircuit, which is located at a switching center serving the multistationline, may interrupt outgoing message transmission to one of the stationson the multistation line for the purpose of making a roll call oftransmitters in order to initiate operation of a transmitter that isprepared to transmit to the switching center. It is desirable to preventthe recording of the transmitter start roll call signals by theteletypewriter recorder of any station or stations that had beenreceiving the message prior to interruption of transmission. The Blanksignal and the pause, which precedes the transmission of the Spacesignal, are employed for blinding the recorders in order that they shallnot respond to the code combinations comprising the selectivetransmitter starting signals. When the roll call has been completed,upon the starting of a transmitter or by canvassing of transmitterswithout finding one that has been conditioned for transmission, anyteletypewriter recorder that had been blinded is unblinded and therebyconditioned to respond to resumed message transmission.

Each station on the multistation line, upon receiving the signal forstarting its transmitter, responds by initiating transmission of amessage if one is awaiting transmission, or by automatically generatingand transmitting the code combination for the character H. The lattersignal indicates that the transmitter has no message awaitingtransmission and authorizes the transmitter start circuit at the remoteswitching center to continue the roll call of transmitters.

In accordance with the present invention, a circuit including electrondischarge components for generating the H signals is activated at eachstation which, having responded to the signal for the starting of itstransmitter, has no message material awaiting transmission. The signalgenerator comprises timing circuits for causing the transmission of themarking and spacing elements of which the signal for the character H iscomprised.

For a complete understanding of the invention, reference may be had tothe following detailed description to be interpreted in light of theaccompanying drawings, in which:

FIGS. l to 4, inclusive, when arranged with FIG, 1 above FIG. 3, andwith FIGS. 2 and 4 disposed at the right of FIGS. 1 and 3 respectively,show the complete circuits of an electronic system for selectingstations in response to address codes each comprising two-character codecombinations.

The format of a message to be directed by the selector mechanism to aparticular station is like that described in the Bacon et al. Patent2,766,318. It comprises an address code for the station that is to beselected, which might be the two-character code BR, followed by aLetters signal which in turn is followed by the carriage return and linefeed signals. It is the function of the carriage return and line feedsignals to condition the teletypewriter recorder at the selectivestation for printing at the beginning of a new line on theteletypewriter stationery. The line feed signal is also employed fordeactivating the selector mechanism so that it shall not select otherstations in response to character code sequences occurring in the bodyof the message that correspond to call-directing or address codes. Theline feed signal may also be followed by a Letters signal to afford abrief time interlude between the line feed signal and the beginning ofthe body of the message. Such interval is not needed in connection withthe operation of an electronic selector mechanism, such as that whichwill be described herein, because the circuits and components provide aninertialess system that is extremely fast in operation. It has been thecustom to provide the interval to accommodate the operation of slowermechanical selector mechanisms so that the deactivate or other functioncould be completed before other signals were received. More than oneaddress code may precede the carriage return and line feed signals, eachbeing followed by a Letters signal to separate the call-directing codesfrom one another. The body of the message, including '6 perhaps theidentification of the called station'as well as the calling station infull text, follows the line feed signal or the Letters signalaccompanying the line feed signal. At the conclusion of the message, thedisconnect signal, comprising the codes for Figures, H and Letters,terminates the message.

In the description which is to follow, the circuit components in theseveral figures of the drawings are identified by three-digit referencenumerals, the first digit of which identifies the figure of the drawingsin which the element or component appears, and the other two digitsdistinguish among the components in each figure of the drawings. Anexception to this rule is the case of certain conductive paths whichextend from one sheet of drawings to another. A conductive path carriesthe same reference numeral from its point of beginning to its terminusor to a junction with one or more other conductors.

Referring now to FIG. 1 of the drawings, the reference numeral 101designates a telegraph station which is a point of origin or transferfor messages to be switched through the switching apparatus embodyingthe present invention to one or more receiving stations. In a switchingsystem such as that shown in the Bacon et a1. patent, station 101 may bea switching center Where messages received from various telegraphstations over lines or trunks are relayed to selected outgoing channelsfor retransmission toward ultimate destinations. For the purpose ofsimplicity, station 101 has been shown merely as a message sourcecomprising a telegraph transmitter 102 of conventional type associatedwith a teletypewriter recorder for producing a home record or monitorcopy of message material transmitted, the teletypewriter beingrepresented symbolically by selector magnet 103. It is pointed out thatthe signal combinations transmitted from transmitter 102 each have sevenelements. The first or start element is always a no-current or spacingelement. The last or stop element is always a current or markingelement. Each of the five intermediate elements may be either marking orspacing depending on the particular character or function to be therebydefined. A positive battery connection 104 for supplying transmissioncurrent is provided at station 101 and the station is connected bytransmission channel 106 to the input of the selector mechanism, theinput being receiving relay 107 which has line conductor 106 connectedto one terminal of its lower winding and the other terminal connected toground. Current flowing through the lower winding during the idlecondition, when no messages are being transmitted, causes the armatureof relay 107 to engage its marking contact. Relay 107 is a polarizedrelay and has its upper or biasing winding included in a conductive pathfrom positive battery through resistor 108, the upper winding of therelay, and resistor 109 to ground. When the transmitter 102 at station101 interrupts the current through the operating winding of relay 107,which is the lower winding, current in the circuit just traced operatesthe armature of the relay to its spacing contact. The spacing contact ofrelay 107 is connected through resistor 109 to ground, and the markingcontact is connected through resistor 111 to a potential differencesource that is negative at 27 volts with respect to ground. All positivebattery indications represent a voltage of 260 relative to ground andall negative battery terminations not otherwise identified hereinrepresent battery connections at volts negative with respect to ground.

Resistors 103 and 109 are of such magnitude that the spacing contact isat a potential of approximately 70 volts positive with respect toground. The armature of relay 107 is connected through a resistor 112 toa junction point 113 to which is connected the cathode of the right-handtriode section of electron discharge tube V3, which is preferably avacuum tube. From the junction point 113 there is also a connectionthrough resistor 114 to the grid of the left-hand triode of tube V3.Both triode sections of tube V3 are biased substantially to cutoff byvirtue of negative battery connections to the grids. The left-handcathode of tube V3 is grounded and the right-hand cathode has negative27 volts in the marking condition of relay 107 and positive 70 volts inthe spacing condition. It follows from this that the negative bias onthe left-hand triode is reduced when relay 107 op rerates to spacingbecause its grid swings towards positive, whereas the negative bias onthe right-hand triode is increased with relay 107 in the spacingcondition because the right-hand cathode swings toward positive. Thebias on both triodes of tube V3 is such that neither triode isconductive in the idle condition of the circuit, with the relay armatureon its marking contact, but the righthand triode is at a lesser negativebias than is the left hand triode. Moreover, the bias on the grids oftube V3 is such that when the armature of relay 107 operates to spacing,the reduction in negative bias on the left-hand triode of tube V3 is notsufiicient to render that triode conductive. Resitor 112 connectedbetween the armature of the relay and junction 113, and capacitor 115con- 'nected from resistor 112 to ground, serve as a filter to preventthe occurrence of voltage fluctuations at junction 113 due to bouncingof the armature of the relay, since the application of multiple pulsesfrom the junction point 113 to the left-hand grid and right-hand cathodeof tube V3 might produce spurious pulses in the output of the tube.

The positive potential that is applied to junction 113 when relay 107goes to spacing is extended over conductors 116, 117, and 118, resistor119, capacitors 121 and 122, and resistor 123 to the grid of theleft-hand triode of'tube V1. Tube V1 is a monostable flip-flop circuitand its purpose is to time a received code combination. Its normalcondition, prior to the response of relay 107 to the start element of acode combination, is that the left-hand triode is cut ofi and theright-hand triode is conducting. In this normal condition, with theright-hand triode conducting, a negative potential of the cathode of theright-hand triode applied through the potential divider comprisingresistors 124 and 126 connected between negative battery and groundholds the right-hand anode at negative potential and this potential isapplied to the grid of the left-hand triode of tube V2, holding thattriode cut ofi. Tube V2 is a free-running multivibrator which hassymmetrical half cycles and provides a timing wave, each cycle of whichhas a duration equal to correctly timed start and selecting elements ofcode combinations.

When a positive pulse is appliedto the grid of the left-hand triode oftube V1 over a previously traced path as relay 107 responds to the startelement of a code combination, that triode is rendered conductive, anddue to the flip-flop connection, including capacitor 125, with the. gridof the right-hand triode the latter triode cuts off. The right-handanode of tube V1 swings positive and holds the left-hand triodeconductive through a clamping connection to the, grid of the lattertriode for the interval in which the right-hand triode remains cut off.Resistor 127 and varistor 128 connected from the junction of capacitors121 and 122 to ground by-passes the negative pulses resulting fromreturn of the armature of relay 107 to the marking contact so that theleft-hand triode of tube V1 is not influenced by that return. The timingcircuit comprising capacitor 125 and resistors associated with the gridof the right-hand triode of tube V1, which time the interval forself-restoration of tube V1 to its normal condition, have such valuesthat that interval is slightly longer than five and one-half elements ofcorrectly timed signals, thereby bringing the time of selfrestorationinto the interval between the middle of the fifth selecting code elementand the beginning of the stop element. An external influence from thefree-running multivibrator comprising tube V2 accelerates the time ofrestoration of tube V1 as will be described hereinafter.

With tube V1 in its oif-normalcondition, the righthand anode of the tubeis positive and this potential is impressed upon the left-hand grid oftube V2, permitting that tube to operate as a multivibrator. Theleft-hand triode conducts in the first half of a received start element,and the right-hand triode conducts during the last half of the startelement and each received significant element. It follows from this thatthe left-hand anode swings negative at the beginning of the startelement and at thebeginning of each selecting element, and swings towardpositive at the middle of the start element and the middle of eachselecting element. The left-hand anode of multivibrator tube V2 isconnected through conductors 129, 130, and 13-1 and branching paths tothe grids of tube V3, each of the branching paths including a blockingcondenser, these condensers being designated by the reference numerals132 and 133. The positive swing is sufiicient to render one or the otherof the triodes of tube V3 conductive momentarily, depending upon theposition of the armature of .relay 107 at the time of occurrence of thepulse. If the armature is on the marking contact, biasing the left-handtriode more negative than the right-hand triode, the latter triode willconduct momentarily. On the contrary, if the armature engages thespacing contact, biasing the right-hand triode more negative than theleft-hand triode, the latter triode will conduct momentarily.

The external control for accurately timing the restoration of tube V1 tonormal condition is derived from the right-hand anode of tube V2 and issupplied through capacitor 134 to the right-hand cathode of tube V1.While tube V2 is operating as a free running multivibrator, a negativecharge on capacitor 125, through which the right-hand triode of tube V1was cut oif, is leaking off and the right-hand grid of tube V1 is risingtoward positive. Since the left-hand anode of tube V2 swings towardnegative at the beginning of each code element and swings towardpositive at the middle of each code element, it follows that theright-hand anode of tube V2 does just the opposite, swinging towardpositive at the beginning of each code element and toward negative atthe middle of each code element. These voltage swings are appliedthrough capacitor 134 to the right-hand cathode of tube V1. Thepositiveswings merely increase the negative bias on the right-handtriode of tube V1. The negative swings, being swings toward thepotential of 'the grid, reduce the bias on the right-hand triode of tubeV1. When the negative cathode swing occurring at the middle of thefourth code element occurs, capacitor has not lost enough of itsnegative charge to permit the cathode swing to render the right-handtriode of tube V1 conductive. However, at the next negative swing of thecathode, which occurs at the middle of the fifth code element, theright-hand grid of tube V1 has risen toward positive sufliciently thatthe negative cathode swing will render the right-hand triode of tube V1conductive. The right-hand anode of tube V1 swings toward negative,cutting off the left-hand triode and holding it cut off until the nextpositive pulse is applied through capacitors 121 and 122.

nation, because relay 107 returns to marking for the stop and restcondition following the fifth code element, applying a negative pulse tocapacitor 121 as the armature of the relay returns to marking. With themonostable flip-flop circuit comprising tube V1 in its normal condition,the operation of the free running multivibrator is suspended and thatmultivibrator remains in its rest condition with the right-hand triodeconductive and the lefthand triode cut off.

It was previously stated that the grids of tube V3 are pulsed positivelyat the middle of each received code element, the right-hand triodeconducting momentarily when 'a marking condition is being received andthe left-hand triode conducting momentarily when a spacing conditionThis pulse should occur at the start transition of the next receivedcode combihand triode of tube V3 includes the primary winding of atransformer 136, and the external anode circuit of the right-hand triodeof the tube includes the primary winding of a transformer 137. Thesecondary winding of transformer 136 is shunted by a resistor and isconnected between ground and an output conductor 138. Similarly, thesecondary winding of transformer 137 is shunted by a resistor and isconnected to output conductor 139. A control pulse is impressed onconductor 139 at the middle of each marking code element by transformer137 and a control pulse is impressed upon conductor 138 by transformer136 at the middle of each spacing code element.

In FIGS. 2 and 4 are shown four gas-filled stepping tubes designated bythe reference numerals 201, 202, 401 and 402. Each of these tubescontains an anode and two different types of cathodes usually designatedas the A and B cathodes, the A cathodes being represented by smallcircles and the B cathodes being represented by triangles, each pointingtoward an individual one of the A cathodes. The A cathodes are allreturned to a point at or near ground, and the B cathodes are connectedselectively to the marking and spacing control conductors 139 and 138.The A cathodes are rest cathodes, and a steady state discharge willexist between any one of the A cathodes and the anode. The B cathodesare transfer cathodes, and will transfer a discharge from one A cathodeto another in one direction only, which is the reason that the tube iscalled a stepping tube. The cathode at the point of beginning of thestepping path is called a reset cathode, and the pulsing of that cathodewill cause the transfer of a discharge to it from any other cathode inthe tube that is then conducting a discharge.

Referring now specifically to FIG. 2, the reset cathode in tube 201 isdesignated by the reference numeral 203. Immediately to the left ofreset cathode 203 is a B cathode which, when pulsed, will cause thedischarge to transfer from reset cathode 203 to A cathode 204. To theleft of this cathode is another B cathode which, upon being pulsed, willcause the discharge to transfer to A cathode 206. To the left of thelatter cathode is a pair of B cathodes, and the pulsing of one of thesewill cause the selective transfer of the discharge to either of the Acathodes 207 or 208. To the left of each of the cathodes 207 and 208 isa pair of B cathodes, and the pulsing of any one of these will cause theselective transfer of the. discharge to any one of four A cathodes.Similarly, there is a pair of B cathodes to. the left of each of thefour A cathodes, and the selective pulsing of any one of these willcause the transfer of the discharge to any one of eight final cathodes.

Reset cathode 203 is connected by conductors 209, 211 and 212 to thecathode of the left-hand triode of tube V4 in FIG. 1. This triode hasits cathode returned to negative battery through resistor 140a, which isthe output load resistor for the triode, its anode having no otherconnection than to positive battery through resistor 14% and its gridbeing biased negatively by a potential divider comprising resistors 140and 141 connected between negative battery and ground- A pulsingconnection fromthe anode of the left-hand triode of character timerflip-flop tube V1 to the left-hand grid of tube V4 includes capacitor142. 7 When the left-hand triode of. flip-flop tube V1 is renderedconductive at. the beginning of the start element of a code combinationand, its anode swings toward negative, a negative pulse. is impressedthrough capacitor 142 to the left-hand grid of tube V4, reducing theconductivity of this triode. Because the output is taken from thecathode, which follows, the potential of the grid, a negative pulse isapplied over conductor 212 to reset cathode 203 to transfer thedischarge tothat cathode. The negative charge on capacitor 142 leaks offand restores the left-hand triode of tube V4 to normal conductivity nearthe end of the received start element, thereby maintainingthe left-handtriode; of tube V4 at reduced conductivity during almost; the en irereceived start element. With a negative potential impressed on the resetcathode for this interval, the pulsing of an adjacent B cathode duringthe interval will not result in the transfer of a discharge from thereset cathode. It will be noted that the B cathode immediately to theleft of the reset cathode 203 is connected to spacing conductor 138which is pulsed about the middle of the received start element. Thispulsing of the B cathode does not result in the transfer of thedischarge to A cathode 204 because the cathode cannot eifect a transferuntil the reset cathode has returned to its normal potential nearground. The reset cathodes in the other three tubes are also connectedto conductor 212, so that the discharge is reset in the tubes during thereception of the start element and is prevented from being steppedduring that element.

The first B or transfer cathode in tube 201 is connected to spacingpulse conductor 138, as is also the second B cathode. It follows fromthis that the first spacing element received after the start elementwill cause the transfer of a discharge from reset cathode 203 to restcathode 204, and the next spacing element will cause another transfer torest cathode 206. From that point on, the transfer cathodes appear inpairs, doubling in number for each step, and comprising one, two andfour pairs. The upper transfer cathode of each pair is connected to themarking pulse conductor 139, and the lower one is con nected to thespacing pulse conductor 138. A marking pulse occurring on conductor 139when the discharge is at rest cathode 206 will cause the discharge totransfer to rest cathode 207, whereas a spacing pulse will cause it totransfer to rest cathode 208. By similar selective transfer in the nexttwo ranks of transfer cathodes, the discharge may be brought to any oneof eight final rest cathodes. The eight final rest cathodes have beendesignated by the character or function signals to which theycorrespond. For example, should the received signal comprise two spacingelements followed by three marking elements, the discharge will be resetto cathode 203 during the start element, will be transferred to restcathode 204 during the first spacing element, to rest cathode 206 duringthe second spacing element, to rest cathode 207 during the first markingelement, which is the third element of the code combination, to restcathode 213 during the second marking element, which is the fourthelement of the code combination, and to final cathode M during the thirdmarking element, which is the fifth element of the code combination. Thefinal cathode M is connected through resistors 214 and 216 to groundconductor 217, and the discharge current flows from ground through thoseresistors and from final cathode M to anode 218, which. is common to allof the cathodes of the tube. All rest cathodes in tube 201 other thanthe final cathodes are connected to conductor 219 which is grounded. Ifthe code combination for carriage return, which consists of the codeelements space, space, space, mark, space, should be received, thedischarge will transfer from reset cathode 203 to rest cathode 204 inresponse to the first spacing element, to rest cathode 206 in responseto the second spacing element, to rest cathode 208 in response to. thethird spacing element, to rest cathode 221 in response to the firstmarking element, and to rest cathode CR inresponse to the final spacingelement.

Consideration will now be given to the response of tube 201 toa signalwhich does not have its first two code elements of spacing nature. Itwill be supposed that the signal for the character W, which is comprisedof' the elements mark, mark, space, space, mark, is received. Since thetransfer of the discharge from reset cathode 203. occurs only inresponse to spacing elements in tube 201, the discharge will remain onreset cathode 203 during the first two elements of the code combinationfor W. In response to the third element, which is of spacing nature, thedischarge will transfer to rest cathode 204, and during the fourth codeelement, which is also of spacing, nature, the discharge will transferto rest cathode 206. During the final code element, which is of markingnature, the discharge will transfer to rest cathode 207. Since the codecombination contains no more code elements, the discharge will notadvance beyond this point, but will be reset during the start element ofthe next received code combination. Thus the discharge will reach one ofthe final cathodes in tube 201 only in response to code combinationscorresponding to one of the eight characters or functions by which thoseeight final cathodes are identified. These are the code combinationswhich have the first and second elements of spacing nature.

Tube 202 has its first transfer cathode 222 connected to spacing pulseconductor 138, and its second transfer cathode 223 connected to markingpulse conductor 139. The remaining transfer cathodes are connected toconductors 138 and 139 in the same manner as those of tube 201, and tube202 provides selective response for those eight code combinations whichhave the first element of spacingnature and the second element ofmarking nature. Similarly, tube 401 is effectively responsive to theeight code combinations having the first and second elements of markingnature, by having first and second transfer cathodes 403 and 404connected to marking pulse conductor 139, and tube 402 is effectivelyresponsive to the eight code combinations having the first element ofmarking nature and the second element of spacing nature, by having thefirst transfer cathode 406 connected to marking pulse conductor 139 andthe second transfer cathode 407 connected to spacing pulse conductor138;

As previously stated, the selector mechanism being described hereineffectuates selections in response to calldirecting codes comprising twocharacters. In order to do this, it is necessary to register and storean indication of reception of any code combination in order to produce acombinational result from two successively received code combinations.The instrumentality used for registering the reception of codecombinations is a single transistor flip-flop circuit shown in FIG. 3.This circuit is fully described and claimed in the previously mentionedB. Ostendorf, ]r., patent and the disclosure of that patent isincorporated herein by reference as if fully 'disclosed in the presentspecification. It will be described in the present specification only tothe extent necessary to convey an understanding of its operation for thepurposes of the present invention.

It will be assumed'that a message is transmitted from station 101,preceded by the address code BR. The code combination for the characterB has its first code element of marking nature and its second codeelement of spacing nature, and the code combination is efiectivelydecoded by stepping tube 402 to establish a discharge at final restcathode B and a resulting flow of current through resistors 408 and 409.The establishment of a discharge through final rest cathode B results ina swing of that cathode toward positive, and the swing is impressed overconductor 411 on the right-hand terminal of capacitor 301. The otherterminal of capacitor 301 is connected through resistor 302 to the base303 of transistor 304, and the base is also connected through resistor306 to ground.

- The collector 307 of transistor 304 is connected. through resistor 308to a negative potential source.

of 820 ohms, and the battery connection serves to bias the collector inthe reverse direction and is perhaps of the order of 26 volts.

a. The emitter electrode 309 is returned to negative potential throughthe load line resistor 311 which is large in ;comparison' with theinternal emitter resistance of the transmitter, and in one embodiment ofthe invention has a 'value of 3 megohms. The negative voltage to whichthe emitter is returned through resistor 311 is the full negativebattery potential of 160 volts. The emitter also has connection throughself-biasing resistor 312 to base The resistor I 308is of relatively lowresistance, perhaps of the order 12 connected through-resistor 143 tonegative battery supplyat 45 volts. In thesteady-state condition of theright-hand'triode of tube V4, the tube is sufliciently conductive toprovide a potential ofv approximately 11 /2 volts negative at thecathode, and this is the potential applied to the right-hand terminal ofvaristor 314.

With the circuit arrangement of the transistor as described, thetransistor is normally held cut off by a small negative bias currentflowing through resistor 311 from the negative battery supply. At thetime of the positive voltage swing applied to conductor 411 by the finalrest cathode B, a positive pulse is applied through capacitor 301 to thetransistor base 303. This pulse efiects no changein the steady-statecondition of the transistor. During the start element of the nextreceived signal combination, the discharge is reset in the steppingtubes 201, 202, 401 and 402. The final cathode B returns to groundpotential, applying a negative-going pulse through capacitor 301 totransistor base 303. The negative transition applied to the transistorbase causes the emitter 309 to be momentarily more positive than thebase and hence positive current flows from. the base to the collector.Feedback action obtained because of resistor 306 to ground causes aneven more negative excursion of the base, and the emitter is pullednegatively along with the base because of the low internalemitter-to-base resistance. Upon reaching theholding potential ofapproximately negative 11.5 volts applied over conductor 317, thevaristor 314 becomes a low impedance source of current supplying a highvalue of emitter current, which in turn sustains a high collectorcurrent. When the circuit has stabilized in the new condition, thecollector-toground voltage is about negative 14 volts, an increase inpotential of approximately 12 volts. The transistor is stable inthis newcondition, which is its ofi-normalcondition.

The collector electrode 307 of the transistor. is connectedthroughresistor 318 to one terminal of a capacitor 319, the other terminal ofwhich is connected over conductor 321 to the junction of resistors 224and 226 connected between the final cathode R of stepping tube 202 andgrounded conductor 217. The same transistor collector electrode is alsoconnected through resistor 322 to one terminal of capacitor 323, theother terminal of which is connected. over conductor 324 to the junctionof resistors 412' and 413, connected in series between final cathode Yof stepping tube 402 and grounded conductor 217 The circuit comprisingresistor 318 and capacitor 319 is a combiner circuit which'is primed bytransistor 304 in preparaiton for. possible reception and decoding ofthe code combination for the character R. Similarly, the circuitcomprising resistor 322 and capacitor 323 is a combiner circuit that isprimed by the same transistor 304 in anticipation of possible receptionof the code combination for the character Y. In the ofi-normal stablecondition, collector electrode 307 of the transistor-is more positivethan it is in the normal stable condition, and the increased potentialcauses a charging current to flowthrough resistors 318 and 322 to chargecapacitors 319 and 323, respectively.

' When thecode combination for the character R has been decoded by tube202, a discharge is established between final cathode R and the anode oftube 202 ,to produce a positive'voltage swing at the junction 'ofresistors 224 and 226. The positive voltage swing, occurring about themiddle of the fifth element'of the code combination, is impressed oncapacitor 319 and is in additive relation to the charge alreadyimpressed thereon by transistor 304. The resultant pulse, appliedthrough crystal diode 326 and conductor 327 to the control grid of tubeV7 renders that tube conductive'for the duration of the pulse. Thepositive screen voltage for tube V7 is removable under circumstances tobe described hereinafter, but it will be assumed tor the present thatthe tube has proper screen voltage and that the positive pulse on itscontrol grid will render it conductive. The anode of tube V71is coupledtube is insensitive to positive potential applied from conductor 117over conductor 143 to the control grid of tube V6 each time the armatureof receiving relay 107 operates to spacing. The negative swing of theanode of tube V7, as it becomes conductive, cuts otf the left-handtriode of tube V5, which renders the right-hand triode conductive. Withthe left-hand triode of tube V cut 01f, the

anode of the tube swings toward positive, thereby raising the potentialof the control grid of tube V6 through a potential divider circuitcomprising resistors 333, 334, 336, 337 and 338. The resulting potentialon the control grid of tube V6 brings it close to the threshold ofconductivity, but the tube is not rendered conductive under the controlof the left-hand triode of tube V5 alone. The anode circuit of tube V6,in which no current is flowing, includes the operating winding of arepeating relay 339 which has its armature held in engagement with themarking contact by current through the biasing winding. A circuitthrough the armature and marking contact of repeating relay 339 includesthe selector magnet 341 of a receiving teletypewriter station 342, whichis the station designated by the adress code BR. When a current flows inthe anode circuit of tube V6, the armature of relay 339 moves out ofengagement with the marking contact, gmpressing a spacing element uponthe selector magnet The code combination for the letter R, in commonwith fifteen other code combinations of the five-unit code, has itsfifth code element of spacing nature. Since tube V7 is renderedconductive momentarily, about the middle of that element, the armatureof receiving relay 107 is engaging its spacing contact, applyingpositive battery over conductor 143 to the grid of tube V6. Since thisis the final code element of a code combination, it would be undesirableto permit receiving teletypewriter 342 to respond to that code element,which it would interpret as the start element of a code combination. Theremoval of the negative holding bias on the control grid of tube- V6 isdelayed by capacitor 343 connected to the junction of resistors 334 and336, the delay extending well into the time of reception of the stopelement following the final code element. When the negative holdingpotential is removed from the grid of tube V6, therselector magnet 341of teletypewriter 342 will respond to all signals received by receivingrelay 107.

' As previously stated, the character timer tube V1 is restored to itsinitial condition at the time of generation of the discharge steppingpulse for the fifth element of a code combination. The return of tube V1to normal con dition involves restoration of conductivity in therighthand triode and cutting oif of the left-hand triode. Asconductivity is restored in the right-hand triode, the anode of thattriode swings toward negative and the negative swing is applied overconductor 144, capacity 147 and resistors 148 and 149 to the grid of theright-hand triode section of tube V4. The function of this triode is toreset any transistor flip-flop circuit that was triggered to itsoffnormal condition at the end of reception of the preceding codecombination. The negative swing of the right-hand anode of tube V1produces a negative swing of the rightflop circuit that is off-normalwill be driven toward negative to restore the flip-flop circuit to itsnormal condition. A capacitor 151 is coupled from the junction ofresistors 148 and 149 to ground, and its function is to delay slightlythe negative-going transition of the grid and cathode of the right-handtriode of tube V4. It should be remembered that the restoration of tubeV1 to normal condition occurs substantially at the same instant that thetransfer of the discharge in one of the stepping tubes to a finalcathode occurs. Refer-ring specifically to the case under consideration,the transfer of the discharge to the final cathode R of tube 202 and thepulsing of capacitor 319, charged by transistor 304, would occursubstantially at that instant. If the restoration of the transistorflip-flop circuit to normal condition were to occur at the same instant,capacitor 319 might discharge to a suflicient extent, that the pulseapplied over conductor 321 would be insufiicient to activate tube V7 andtrigger tube V5. By restoring elf-normal transistor flip-flop circuitafter the associated capacitor has been pulsed, the charge on thecapacitor will not be lost.

It will also be remembered that the triggering of a transistor flip-flopcircuit in response to a received code combination does not occur at thetime of transfer of the discharge to the final cathode representing thatcode combination, at substantially the midpoint of the last code elementof the code combination, but occurs instead at the resetting of thedischarge in the stepping tube during the start element of the nextreceived code combination. Thus the sequence of events, when the lastcode element of a code combination hasv been identified, is to pulse thecapacitor of a combiner circuit associated with a transistor flip-flopcircuit that was triggered during the start element of that codecombination as a result of a selection made in response to the precedingcode combination, then to restore that transistor flip-flop circuit, andin the start element of the next code combination to trigger thetransistor flip-flop circuit representing the code combination justcompleted.

It will be noted with reference to FIG.2 that there, is no connectionfrom the final cathode R of tube-202 to a single transistor flip-flopcircuit. The significance of this is that among-the teletypewriterstations controlled by the decoding circuits contained in FIGS. 2 and 4,there is none having the letter R as the first character of its addresscode. If there were a station so designated, a single transistorflip-flop circuit would be connected to the final cathode R, and thattrigger circuit would be triggered offnormal upon the resetting of thedischarge in tube 202 during the start element of the code combinationfollowing the address code BR, which is a Letters code combination.

That single transistor flip-flop circuit would be restored to normalafter the middle of the last selecting element of the Letters signalthrough the agencyof timer tube V1 and the right-hand triode of tube V4,as described. Thus the Letters signal is not used as thefirst characterof any twocharacter code so that there is no connection from the finalcathode representing the Letters signal to a single transistor flip-flopcircuit. From this it follows that at the end of the Letters signal noneof the single transistor flip-flop circuits is off-normal.

If a message is addressed to more than one station served by the samestation selector unit or circuit, the

address codes forfthe stations addressed, each comprising atwo-character address code followed by the Letters signal, will bereceived by relay. 107 For example, the address code BY might'bereceived. The transistor flipfiop circuit 304 will be triggeredcit-normal in response to the character B to prime the combiner circuitcomprising resistor 322 and capacitor 323, and the capacitor will bepulsed in response to the signal representing the character Y,delivering a pulse throughv crystal diode 325 and over conductor 330 toactivate momentarily the control tube of a station selector circuit (notshown). The address codes include a group code, such as code GX. Aflip-flop sage.

circuit including transistor 344 is arranged to be triggered off-normalover conductor 346 which is connected to final cathode G in tube 202. InFIG. 3, the transistor 344 is arranged to prime three combiner circuits347, 348, and 349. The capacitor in combiner circuit 347 is connectedover conductor 351 to be pulsed when a discharge reaches the finalcathode X of tube 402 in response to the code combination for thecharacter X. This combiner circuit, upon being pulsed, extends the pulseto conductor 327, thereby selecting station 342 in addition to any otherstation designated by the address code GX and served by the sameselector circuit or another similar selector circuit in the system. Inthis way, a message may be routed to two or more stations in response toan address code jointly designating them. The other two combinercircuits that a are primed by transistor 344 are not pulsed in responseto the character X, because their capacitors have other connections.Specifically, combiner circuit 348 has its capacitor connected overconductor 352 extending to the resistive path that becomes conductivewhen a discharge transfers to final cathode A in tube 401, the combinercircuit 348 combining the codes G and A, and the capacitor associatedwith combiner circuit 349 is connected over conductor 353 to theresistive path associated with final cathode J in the same steppingtube, combiner circuit 349 combining the codes G and J.

The last of the address codes is followed by the carriage return andline feed signals which are effective, in the teletypewriters that havebeen selected to receive the message, to advance the stationery in theteletypewriter to the point of beginning of a new line for the recordingof the message to follow. "Hie combinations 'for carriage return andline [feed are impressed directly from relays such as relay 107 andtubes such as tube V6-to control the selected teletypewriters. They alsoaffect selections in tubes such as tubes 201 and 202, respectively, andthe selection effected by the line feed combination is used to blind thestation selector circuits against further response to address codesappearing randomly in the text of the mes- There is no connection to theresistor path associated with final cathode CR of stepping tube 201, towhich a discharge is transferred inresponse to the carriage returnsignal. There is a connection, however, to the final cathode LP instepping tube 202, and this connection extends over conductor 227,capacitor 354 and resistor 356 to the left-hand grid of tube V8. Thistube has connections establishing a bi-stable flip-flop circuit.

fT'he tube V8, together with the tube V9, comprises an lections ofstations in response toaddress codes. 'In the activate condition, theleft-hand triocle of tube V8 is conducting, and the right-hand triode iscut off. This causes the positive potential of non-conductive right-handanode of tube V8 to be applied over conductor 357 to the screen grid oftube V7. Earlier in this description it wasrassumed that this tube hadproper screen voltage 'for its operation. That screen voltage isobtained from tube V8. Since the left-hand triode of tube V8 isconducting, the positive pulse applied toits grid from final cathode ifof tube 202 when a discharge is transferred to that cathode, has noeffect on tube V8. When the discharge is reset in tube 202 about themiddle of the startelement of the next received code combination, anegative pulse is applied over conductor 227 and through capacitor 354to cut ofi the left-hand triode of tube V8. The flip-flop connectionscause the right-hand triode to become conductivc and the anode'swingstoward negative, removing the positive screen voltage from tube V7 andthereby rendering that tube insensitive to control by its control grid.During reception of the message, the stepping tubes jcontinue to decodereceived code combinations, stepping discharges to their final cathodesselectively, triggering associated single transistor flip-flop circuits,and pulsing tube V7 and corresponding tubes when sequences of two codecombinations correspond to address codes in response to which thosetubes are to be pulsed. The pulsing of the control grids of the tubes isineffective be: cause of removal of the screen voltage, and thoseunselected stations cannot be inadvertently selected.

Each time that a Figures code combination is received, a singletransistor flip-flop circuit including transistor 35.8 is triggered,during the start element of the next code combination, by a negativepulse impressed through capacitor 359 from conductor 361 which isconnected to the final cathode FIGS. in tube 401. This single transistorflip-flop circuit, upon being triggered, primes acode comfbiner circuitcomprising resistor 362 and capacitor 363 associated with the grid oftube V9, and capacitor 363 is connected over conductor 364 to the finalcathode H of tube 201. The only circumstance under which the Hcodecombination properly follows the Figures code combination is theend-of-mcssage or disconnect signal. Accordingly, any occurrence of theFigures signal in the text of the message will merely result'in thepriming of the combiner circuit associated with the grid of tube V9, andin response to the next succeeding code combination, transistor 358 willbe restored. However, when the H signal follows the Figures signal, thepositive pulse applied from the final cathode H of tube 201 throughcapacitor 363, which is charged from ofiE-normal transistor circuit 358,will render. tube V9 conductive momentarily. Its anode, which isconnected through capacitor 366 and resistor 367 to the right-hand gridof tube V8, will swing negative momentarily, cutting off the right-handtn'ode of tube V8 and restoring conductivity to the left-hand triode.With the righthand triode cut off, the screen voltage will be restoredto tube V7 and corresponding tubes, thereby effecting reactivation ofthe station selecting cir- .cuits.

[be started in response to transmitter start signals from station 101 isdesignated by reference numeral 153, and it may be a conventional tapetransmitter controlled by clutch magnet 154. The circuit of: clutch magn15 is closable by relay 156, which is operable upon completion of itsoperating circuit over either of two paths. Common to the two paths arethe contacts 157 that are usually designated as sixth pin contacts, andthese contacts close when a perforated tape containing message materialto be transmitted is placed in the tape transmitter. Also common tothetwo paths is the swinger of switch or key 158, which is manuallyoperable to select one or another of two possible energizing circuits.The key 158 is positionable in one or another of two positionsdepending; upon the degree of urgency of the message, which has beendescribed in the Bacon-Branson-Knandel- Locke Patent 2,871,286 aspriority or non-priority rating. As disclosed in that patent, onetransmitter start code may be assigned to a transmitter for prioritymessage p ck up and another maybe assigned to the same transmitter tornon-priority message pick up. The "circuit which originates transmitterstart signals invariably first transmits a roll call of prioritytransmitter start codes in a cycle of operation, and transmits nonpriority transmitter start signals-only if it makes a completeroll callof transmitters for priority message trafiic without finding any suchtraffic awaiting transmission. It will be assumed that tape has beenplaczedin transmiter 153, causing the c o ure of siathp n contacts 5that key has been closed to the left-hand contact as shown, which willbe presumed to be the non-priority condition. This selects an energizingpath for relay 156 which includes the armature 3 and front contact ofrelay 160. The transmitter 153 will -be started in due courseautomatically, and in response to its assigned priority roll call signalwithout further supervisory attention, as will now be described.

It will be presumed that station 342 is receiving a message, and thatbefore the completion of the message a Blank signal is received fromstation 191. This signal has all of its selecting elements of spacingnature. At the time of reception of the Blank code combination, theselector mechanism is unable to determine whether the Blank signal isthe beginning of a transmitter start pattern or whether the signal wasinadverently included in the message transmitted from station 191. Ineither case, the selector mechanism decodes the signal, and the decodingoperation results in the transfer of a discharge to the final cathodeBLK in stepping tube 201. The potential of that cathode swings towardpositive, correspondingly extending a positive voltage swing overconductor 228 to the grid of the left-hand triode of tube Vifi in FIG.3. Normally, the left-hand triode of tube V is held cut off by thepositive potential applied to its cathode through a resistor. Theexcursion of the grid toward positive causes the left-hand triode toswing from cut-off to full-on-condition, and its anode accordinglyswings toward negative, as indicated by a fragmentary voltage graphabove and to the left of tube V10. Prior to the activation of theleft-hand triode of tube V16, a capacitor 384 associated with thecircuit of the left-hand anode is charged to a potential determined by avoltage divider comprising resistors 385, 386 and 387, a crystal diode388, and a potentiometer 389 supplying a potential which is positivewith respect to ground. Crystal diode 3 38 is connected in the lowimpedance direction with respect to the polarities applied to its twoterminals. FOL lowing the sudden negative transition of the left-handanode of tube V10 as the left-hand triode becomes conductive, capacitor334 begins to discharge exponentially at a rate determined particularlyby the values of resistor 386 and capacitor 384. Due to the clampingaction of the crystal diode 388, the voltage at the junction of resistor387 and the crystal diode does not change until the voltage on thecapacitor 384 is more negative than the clamping voltage determined bythe potentiometer 389. When the point is reached at which the crystaldiode 388 no longer passes a significant amount of current and,accordingly, is in the high impedance condition, a fraction of theexponential voltage on capacitor 334 exists at the junction of resistor337 and crystal diode 383, and is impressed through capacitor 390 on thegrid of the righthand triode of tube V10. This triode is normallyconductive, holding its anode at a potential less than full positivevoltage. The negative swing of the right-hand grid of tube Vii} isaccompanied by a positive swing of the anode as conductivity in the tubeis reduced, and a positive pulse is applied through capacitor 391 to thecathode of the left-hand triode of tube V11. It will be apparent fromthe foregoing that the right-hand triode of tube V10 does not undergo asubstantial reduction in conductivity until after a delay intervaldetermined mostly by the time constant of resistor 386 and capacitor 384and the clamping voltage. The amount of delay can be varied by changingthe clamping Voltage which is under the control of the potentiometer389. The nearer the clamping voltage is to ground, the longer will bethe delay, providing the clamping voltage is more positive than theanode voltage of the left-hand triode of tube V10 when that triode is inthe conducting condition. The voltage change at the anode of theright-hand triode of tube V10 is indicated by the fragmentary voltagegraph above and to the right of tube V10.

The left-hand triode of tube V11 is normally conductive, and thepositive pulse applied through capacitor 391 to its cathode is theequivalent of a'negative pulse on its grid so that the left-hand triodeof tube V11 cuts off, its anode swinging toward positive. The left-handanode of tube V11 is connected through a potential divider circuit tothe grid of the right-hand triode, and with the lefthand triode cut 01f,the right-hand triode is rendered conductive, its anode swinging towardnegative. These anode changes are indicated by accompanying voltagegraphs. The right-hand anode of tube V11 is connected over conductor 392to the grid of tube V6, and with the right-hand triode of tube V11conductive, and while that triode [remains conductive, tube V6 isrendered unresponsive to signals repeated by relay 107. This representsthe blinding of teletypewriter 342 that has been receiving a message, sothat it will not respond to the transmitter start codes that may beexpected to be received after a pause of a duration sufiicient to causethe cutting off of the right-hand triode of tube V10 and the pulsing ofthe left-hand cathode of tube V11.

Before proceeding with a description of operations resulting fromreception of a proper transmitter start pattern, consideration will begiven to the conditions that result when the Blank signal is merely anidle signal in the message and is not followed by a pause of the properduration. It will be remembered that the discharge is transferred from afinal cathode of any of the stepping tubes to the reset cathode duringreception of the start element of the next code combination. If theBlank signal, which causes the left-hand triode of tube V10 to becomeconductive, is followed almost immediately by another and different codecombination, the discharge will be transferred from the final cathodeBLK of tube 201 to the reset cathode. This will cause the left-handtriode of tube V10 to be cut oft before capacitor 384 has timed out andcut 011 the right-hand triode of the tube. It follows from this thatonly in response to a rest or pause of a predetermined minimum duration,usually of the order of one second, will the teletypewriter receivingcircuits be blinded and other preparations be made for reception of thetransmitter start codes. Conductor 392 is connected in multiple to alltubes corresponding to tube V6, so that the blinding condition isapplied to all of the receiving teletypewriters.

Tube V11 has its two triode sections connected as a monostable flip-flopcircuit. Prior to the pulsing of the left-hand cathode through capacitor391, the left-hand triode is conductive and the right-hand triode is cutoff. When the tube reverses and the right-hand triode becomesconductive, its anode applies a negative pulse through a couplingcapacitor 398 to the left-hand grid to hold the left-hand triode cut offfor an interval. There is also a connection from the right-hand anode oftube V11 through resistor 393 to the grid of the left-hand triode oftube V12. This tube has cross-connections between its anodes and controlgrids, establishing a bi-stable, flipfiop circuit, and the left-handtriode is normally conductive. The negative-going swing applied to itsleft-hand grid from the right-hand anode of tube V11 is preparatory,reducing somewhat the conductivity, but being insuflicient to cut offthe left-hand triode or cause the right-hand triode to becomeconductive.

The timed pause following the transmission of the Blank signal fromstation 161 is ended by the transmission of a Space code combination,and the circuit that controls the transmission of the transmitter startpattern transmits the Space signal before tube V11 times out andrestores itself to the initial condition. The Space code combinationresults in the transfer of a discharge to final cathode SP in steppingtube 201, swinging that cathode toward positive. The positive voltageswing is applied over conductor 229 and capacitor 394 to the left-handcathode of tube V12. The positive swing of the cathode has the sameeifect as a further negative swing of the grid would have, and issufiicient to cut ofi the left-hand triode, thereby causing theright-hand triode of tube V12 to become conductive. tive, and itspotential is applied to the left-hand grid of the tube to hold theleft-hand triode cut off. Fragmentary voltage graphs above tube V12represent the voltage changes at the respective anodes. There is aconnection from the right-hand anode of tube V12 through resistors 395and 396 to the left-hand grid of tube V11 to hold that grid negative andthe left-hand triode cut off as long as tube V12 remains off-normal eventhough tube V11, which is monostable, should time out and seek torestore itself to initial condition. Tubes V11 and V12 remain in theoff-normal condition throughout the roll call of transmitters and untilthe reception by relay 107 of a Letters sign-a1, which indicates that atransmitter has been started or that a full roll call has been madewithout finding a transmitter having material awaiting transmission. TheLetters signal is decoded and results in the transfer of a discharge tofinal cathode LTRS of tube 401. This causes a positive voltage swing tobe applied over conductor 418 and capacitor 330 to the cathode of theright-hand triode of tube V12. The positive swing of the cathode causesthe cutting oil? of the right-hand triode, which in turn causesrestoration of conductivity in the left-hand triode. With the right-handtriode cut off, the lock on tubeV11, applied to the left-hand grid, isremoved. At the same time, the negative swing of the lefthand anode oftube V12, as the left-hand triode becomes conductive, is impressedthrough capacitor 397 on the right-hand grid of tube V11. That triodewill be cut off, applying a positive pulse through capacitor 398 to thelefthand grid of the tube to restore conductivity to the lefthandtriode. The forcible cutting off of the right-hand triode of tube V11through capacitor 397 operates as a quick reset arrangement to restoretube V11 to normal condition even though the tube may not have timed outto its self-restoring condition. The restoration of tube V11 results inthe ire-establishment on the grid of tube V6 of a potential that willenable it to follow signals repeated by relay 107, assuming that station342 had been receiving a message when the transmission of transmitterstart signals was begun. The unblocking of tube V 6 representsunblinding of the receiving station.

It will be observed that although tube V11 is a monostable flip-flopcircuit, there is associated with it a lock for precludingself-restoration and a reset arrangement for forcing restoration if theself-restoring circuit has not timed out. The reason for imposingexternal controls upon tube V11 is that it may be required to beoffnormal for intervals that vary considerably in duration.

If the transmission of a single startcode results in they successfulstarting of a transmitter, the Letters signal, indicating the end of theroll call, may be received before the tube V11 has self-timed forrestoration, and the quick reset arrangement for overriding theself-restoring circuit is necessary.' On the other hand, it may happenthat a complete roll call is made, and that the tube V11 times outbefore the completion of the roll call and reception of the Letterssignal. In that event, the lock applied to the left-hand grid of tubeV11 is necessary. Finally, there is the possibility that a Blank signalmight be transmitted from station 101, following which transmissionmight be suspended. This would be an abnormal condition, but it couldoccur. This would not be a transmitter start pattern, and there would beno Space signal to operate tube V12 otf-normal. It would not bedesirable under that circumstance to maintain the receiving circuitsblinded for an indefinite interval, since transmission might be resumedat any time. Accordingly, the self-restoring feature of tube V11 permitsit to restore and unlock the receiving circuits upon failure to receivea Space signal within a short interval after it should have beenreceived. In actual practice, tube V11 is operated off-normal by tube Vabout .25 second after the reception of the Blank signal, the Spacesignal is received to operate The right-hand anode swings toward negatube V12 oif-normal about 0.5 second after reception of the Blanksignal, and if the Space signal is not received, tube V11 self-restoresabout .75 second after reception of the Blank signal.

Prior to reception of the Letters signal, one or more single charactercodes, each representing the call signal of a transmitter, are received.It will be assumed that one of the code combinations received representsthe character A, and results in the transfer of a discharge to the finalcathode A of tube 401. The potential of this cathode swings towardpositive, and the raised potential is extended over conductor 419 to thecathode of crystal diode 159, the anode of which is connected to thegrid of the left-hand triode of tube V13. Prior to reception of theSpace signal, a potential divider system connected to the cathode of acrystal diode 161, including the lefthand triode of tube V12, which isthen conductive and has its anode connected over conductor 399 andresistor to the cathode of crystal diode 1-61, and also includingresistor connected from the cathode of diode 161 to negative battery,maintains the left-hand grid of tube V13 sufliciently negative to holdthe left-hand triode cut oif. Upon the reversal of tube V12 in responseto the Space signal and the raising of the potential of its lefthandanode to a more positive value, the grid of the lefthand triode of tubeV13 becomes less negative, but the tube is still cut off. The positiveswing applied to the cathode of crystal diode 159 makes the grid of thelefthand triode of tube V13 still less negative, and that triodeconducts.

The anode circuit of the left-hand triode of tube V13 includes thewinding of relay 160 which operates. At the armature 3 and front contactof relay 161), an energizing circuit for relay 156 is completed, thecircuit being traced from ground through the sixth pin contacts 157, key158 in the non-priority position, the armature 3 and front contact ofrelay 160, and the winding of relay 156 to battery. Relay 156 locksthrough its armature 2 and front contact and the sixth pin contacts 157to ground, the locking path shunting the armature 3 and front contact ofrelay 160. At its armature 3 and front contact, the relay 156 completesthe energizing circuit for transmitter clutch magnet 154 which initiatestransmission.

The transmission path that is controllable by distributor face 155extendsfrom negative battery through the rest segment, and through othersegments that may become connected thereto selectively under the controlof tape-sensing contacts (not shown), the distributor brushes andcollector ring, conductor 177 including dotted section 180, thedistributor face or other type of transmitting mechanism of any otherone or more transmitters that are controllable from the selectormechanism in FIGS. 2 and 4, being represented by the transmittercontacts 178, and resistor 176 to the grid of tube V15. The dottedsection 180 represents transmitters additional to the transmitters 153and 178. The grid of tube V15 is also connected to the anodes of tubeV14 which is normally conductive, and the function of which is togenerate an answer-back character, as will be described hereinafter. Thecombination of potentials applied to the grid of tube V15 when none ofthe transmitters is in operation, com prising the negative batteryconnection applied to conductor 177, and those applied through resistivepaths from positive battery and from ground through resistors to thegrid of tube V15, holds the tube cut 011. The anode circuit of tube V15includes the operating winding of transmitting relay 173, and with tubeV15 cut oif, no current flows in the operating winding, but the currentthrough a biasing winding of the relay holds the armature on the markingcontact to apply marking po tential to transmitting conductor 174. \Vhenthe negative battery connection is removed from the grid of tube V15 asthe brushes of distributor 153 pass over the start segment and oversegments selectively conditioned for transmission of a spacing element,tube V15 becomes conductive, and current flows through the operatingwinding of relay 173 to move the armature to the spacing contact whichis grounded, thereby initiating the transmission of a spacing element.Reconnection of the negative potential to conductor 177 results in thecutting off of tube V15 and the reoperation of the armature of relay 173to its marking contact.

As is fully disclosed in the Bacon-Branson-Knandel- Locke application,the reception at the control station of transmission from a transmitterthat is started in response to a roll call signal results in thetransmission by the transmitter start circuit at the control station ofthe Letters signal, which is indicative of the end of a roll call, andis efiective when received by relay 107 and tube 4&1 to unblind anyreceivers that had been blinded against response to the transmitterstart codes. The unblinding as applied to station 342, has beendescribed previously, and is necessary in order that those receivers maybe conditioned to resume message reception. After the reception of thecode signal combination for the letter A, as a transmitter start signal,which started distributor 153, as assumed, the resetting of thedischarge in tube 461 during the start element preceding the Letterssignal which is assumed to follow the A signal, results in the removalof the positive potential from the cathode of crystal diode 159 and thecutting off of the left-hand triode of tube V13, thereby releasing relay169. Since relay 156 locked in the operated condition, there is nofurther need for relay 160 to remain operated. When the transmission ofthe message from transmitter 153 is completed and the message tape runsout, the sixth pin contacts 157 open, unlocking relay 156, whichreleases distributor clutch magnet 154 to suspend outgoing transmission.

in any roll call of transmitters, the priority transmitter start codesare transmitted before the nonpriority roll call, the latter roll calloccurring only if no transmitter has been conditioned for prioritytransmission. The priority code for transmitter 153 is the codecombination for the character S. Accordingly, this code combination istransmitted among other priority start codes preceding the transmissionof the code for the character A, and when it is decoded, the dischargein tube 402 transfers to final cathode S, swinging toward positive theconductor 421 which is connected to the cathode of crystal diode 163.This diode has its anode connected to the grid of the right-hand triodeof tube V13, and there is also associated with that grid a crystal diode164 which has its cathode connected to the same potential dividernetwork as the crystal diode 161. Accordingly, the preparatory enablingvoltage is applied to the right-hand grid of tube V13 at the same timeas the application of the enabling voltage to the left-hand triode whentube V12 is triggered. It follows that the right-hand triode of tube V13becomes conductive in response to the code for the character S andoperates priority transmitter start relay 165. Relay 156 is operablethrough the armature 3 and front contact of relay 165 when priorityswitch 158 is in the priority position. Thus the switch 158 determineswhich of the two transmitter start codes can cause the operation ofrelay 156. When there is no tape in the transmitter, contacts 157 willbe open, and relay 156 cannot be operated by either of the relays 160and 165.

As disclosed in the Bacon-Branson-Knandel-Locke Patent 2,871,286, thetransmission of successive transmitter start codes of a roll call isdependent upon a response from each of the called transmitters. When thetransmitter responds with message transmission, the reception of thattransmission at the control station 101 ends the roll call bytransmitting a Letters signal from the control station.

When a transmitter is not conditioned to transmit a message, the codecombination for the character H is automatically generated andtransmitted to the control station, and the reception of that codecombination at the control station results in reactivation of thetransmitter start circuit for the transmission of the next transmitterstart code in the roll call. The present invention includes anarrangement for generating and transmitting an H signal when atransmitter is not conditioned to transmit, and the generation of the Hsignal is controlled by the character timer comprising the circuit oftube V1. For the purpose of initiating the generation of the H signal,each of the relays and is arranged to connect negative battery throughits front contact and armature 1 to conductor 167, from which a pathcontinues over conductor 168 to one terminal of capacitor 169, the otherterminal of which is connected through resistor 171 to the grid of theright-hand triode of tube V1. The application of a negative pulsethrough capacitor 169 to the right-hand grid has the same efiect on tubeV1 as the application of a positive pulse on the left-hand grid fromrelay 107, through capacitors 121 and 122, namely, the triggering of thecharacter timer tube, cutting off the right-hand triode and renderingthe left-hand triode conductive. With the left-hand triode of tube V1rendered conductive, its anode swings toward negative, and the negativeexcursion of the anode is extended over conductor 170 and throughresistor to the grid of the right-hand triode of tube V14. This resistoris shunted by resistors 179 and 181 in series, and from the junction ofthose two resistors, there is a connection to ground through capacitor182. The righthand grid of tube V14 is also connected through a resistorto negative battery. The effect of the network comprising resistors 185,179 and 181 and capacitor 182 is to delay the application of thenegative voltage swing to the 'righvhand grid of tube V14 for aninterval equal to one-half of a permutation code character element afterthe triggering of character timer tube V1. When the negative voltagebecomes elfective on the right-hand grid of tube V14, its cuts ofi theright-hand triode, and this cutting off coincides substantially with theend of the first half cycle of multivibrator tube V2 which is set inoperation by the triggering of tube V1.

The negative potential applied to conductor 167 by relay 166 or 165,which triggers the character timer tube V1 over conductor 168, is alsoextended over conductor 172 to a potential divider associated with thelefthand grid of tube V14 and including resistor 188. The left-handtriode of the tube is normally conductive. After closure of contacts onrelays 169 or 165 and providing relay 156 doesnt operate, the negativepotential applied over conductor 172 cuts it olf. The tube remainsconductive in its right-hand triode until it is cut 011 after aninterval of one-half of a code element, as has been described, and sincethe two anodes of the tube are connected together and to the grid oftube V15, that grid does not swing positive and render tube V15conductive until the right-hand triode of tube V14 cuts off. With tubeV14 cut off and tube V15 rendered conductive, the relay 173 operates tospacing, impressing a spacing condition on outgoing transmission channel174. This is the mark-to-space transition initiating the start elementof the H signal.

The right-hand anode of character timer tube V1 is connected throughconductor 183, capacitor 184, and resistor 186, to the left-hand grid oftube V16 which has circuit connections establishing a monostableflip-flop circuit. This tube is normally conductive in the right-handtriode, and its left-hand triode is normally cut oii. A positive pulseapplied through capacitor 184, when the righthand triode of tube V1 cutsofi, results in the activation of the left-hand triode of tube V16 andthe cutting off or" the right-hand triode. The circuit constants aresuch that tube V16 will time out and restore itself, without anyexternal control, in an interval slightly longer than the duration of3.5 code elements. Fragmentary voltage graphs to the left of tube V16show the voltage changes of the grid and anode of the left-hand triodeof tube V16.

An external control for tube V16 is supplied by element timer tube V2.Each time a positive-going swing is applied over conductors 129, 130 and131 from the lefthand anode of tube V2 to pulse the grids of tubes V3, apulse is impressed from the junction of conductors 130 and 131 throughcapacitor 189, resistor 190 and capacitor 191 to the right-hand grid oftube V16. As previously stated, the first pulse occurs one-half of acode element after the triggering of character timer tube V1 and becauseof the free-running operation of multivibrator tube V2, subsequentpulses occur at the ends of succeeding intervals each having a durationof one character element. Accordingly,'the right-hand grid of tube V16is pulsed positively at 0.5, 1.5, 2.5 and 3.5 code elements after thetriggering of character timer tube V1 and tube V16. At the end of 2.5code elements, tube V16 has not timed out suificiently. to respond tothe positive pulse, but the next succeeding positive pulse, at the endof 3.5 code elements, is eifective to restore tube V16 to its initialcondition, with the left-hand triode cut off and the right-hand triodeconductive. As the left-hand triode cuts off, its anode swings towardpositive and applies a positive pulse through capacitor 187 to thelefthand grid of tube V14. This pulse renders the left-hand triode oftube V14 conductive, thereby cutting ofi tube V15 and restoring relay173 to the marking condition. Since tube V15 was rendered conductiveone-half code element after character timer tube V1 was triggered and iscut off 3.5 code elements later, it has been conductive for 3 codeelements during which the relay 173 transmits a spacing condition, whichis interpreted by a remote teletypewriter receiver as a start element,and the first and second selecting code elements of spacing nature. Thecutting oif of tube V15 initiates the transmission of a marking element.

The positive pulse through capacitor 187 becomes sufiiciently dissipatedby the end of a character element that the left-hand triode of tube V14again cuts 01f. This causes tube V15 to become conductive and operaterelay 173 to spacing after one marking element, and the spacingcondition is interpreted by the remote receiver as the fourth element ofa code combination. One character element later, which is a total of 5.5code elements after character timer tube V1 was triggered, it restoresto normal, thereby applying a positive potential to the righthand gridof tube V14 to render that triode again conductive, cutting ofi tube V15and restoring relay 173 to marking. A remote receiver interprets this asthe fifth element of a received code combination of marking naturebecause tube V14 did not initially cut off until one-half of a codeelement after the character timer tube V1 was triggered. The restorationof conductivity in tube V14, as character timer tube V1 restores, occurs5 code elements after tube V14 initially cut off. A remote receiverinterprets the received sequence of signals as aw start element, twoselecting spacing elements, a selecting marking element, and a selectingspacing element, followed by a continuous marking condition which itinterprets as a fifth selecting element of marking nature, followed by astop or rest condition. This is the code combination for the characterH, which is transmitted by relay 173 under the control of tubes V14,V15, and V16. The potential changes at the anodes of tube V14 areindicated by the voltage graph above the tube. The transmitter startcircuit'at the control station transmits a start code for the nextsucceeding transmitter in the roll call sequence in response toreception of the H signal. If the next start code received by relay 107also pertains to a transmitter that is controlled by the selectorcircuit in FIGS. 2 and 4, and such transmitter also has no messageawaiting transmission, the cycle of operation of tube V 14, V15 and V16will be repeated.

It will be apparent that either of the relays 160 and 165, uponoperating, invariably triggers the character timer tube V1 to initiateand control the generation of 24 the H signal. As previously stated, theH signal is to be transmitted only when a transmitter that is called hasno message tape awaiting transmission. The suppression of transmissionof the H signal is eifected by the transmitter start control relay 156.It will be noted that the positive'battery for tube V14 is suppliedthrough the armature 2 and front contact of either of the relays and165, or through armatures and contacts of corresponding relaysassociated with other transmitters, such as the transmitter 178, backcontact and armature 1 of relay 156, and corresponding armatures andback contacts of relays corresponding to relay 156 that are h1- volvedin the starting of any other transmitters, such as transmitter 178, andconductor 193, in which the dotted portion represents such otherrelayrcontacts, to the anodes of the tube. When relay 156 is operated,the anode supply battery is disconnected from the anodes of tube V14.Although armature 2 of relay 160 or seeks to supply plate potential fortube V14, relay 156 prevents the con nection of that potential. Thustube V14 is normally without supply voltage and remains so while itsgrids are pulsed in the manner to cause the generation of an H signal.Without any anode potential for tube V14, the grid of tube V15 is notaifected by tube V14, and tube V15 remains cut 0E so that relay 173 doesnot operate. It is necessary to prevent tube V14 from controlling relay173 becauseit would garble the signals generated by distributor 153which is set in operation when relay 156 is operated.

Although a specific embodiment of the invention'has been shown in thedrawings and described in the foregoing specification, it will beunderstood that'the invention is not limited to the specific embodimentsbut is capable of modification, substitution and rearrangement of partsand elements without departing from the spirit of the invention.

What is claimed is:

1. An electronic switching device comprising a bistable element havingfirst and second stable conditions of operation, a second element, acontrol means connecting said bistable element and said second element,and iustr'umentalities in said means for causing said second element tobe bistable in operation when said histable element is in said secondcondition and to be monostable in operation when said bistable elementis in said first condition.

2, An electronic switching device comprising a first flip-flop circuithaving a first and a second condition of stability, a second fiiplopcircuit having a first and a second condition of stability connected tosaid first flipfiop circuit, means in said connection for operating saidsecond flip-flop circuit from its first condition to its secondcondition for an interval and subsequently restoring said secondflip-flop circuit to its first condition when said first flip-flopcircuit is in its first condition of stability, and a control meansbetween said first and second flip-flop circuits to preclude therestoration of said second fiipflop circuit to its first condition whensaid first flip-flop circuit is in its second condition.

3. A telegraph code signal generating circuit having a self-restoringelectron discharge timing circuit, said discharge circuit having a firstand a second condition of operation, said generating circuit includingmeans for operating said discharge circuit to its second condition totime an interval, an oscillator circuit operable while said dischargetiming circuit is in its second condition, an electron discharge tubehaving a plurality of control grids, a control means connecting saidtiming circuit to one of said grids to suppress current fiow in saidtube upon operation of said timing circuit to its second condition, asecond self-restoring timing circuit operable from a first to a secondcondition by'said first-mentioned timing circuit, a control meansconnecting said oscillator circuit to said second timing circuit tocontrol the instant of restoration thereof to its first condition. and acontrol 25 means connecting said second timing circuit to another ofsaid plurality of grids to produce code signal transitions in the outputof said tube.

4. A telegraph station selector system having a code signal receivingcircuit, a decoding circuit in said receiving circuit having a pluralityof output terminals and adapted to apply a pulse to one of saidterminals in response to reception of a particular signal by saidreceiving circuit, a normally non-conducting electron device having agrid and an anode, a control means connecting said one terminal of saiddecoding circuit to said grid of said normally non-conducting electrondevice to cause conduction in said normally non-conducting device uponpulsing of said one terminal, a normally conducting electron devicehaving a grid and an anode, a pulse time delay circuit, and a secondcontrol means connecting said anode of said normally non-conductingdevice to said grid of said normally conducting device including saidpulse time delay circuit to suppress conduction in said normallyconducting device when said particular signal has been received for apredetermined time.

5. A telegraph station selector system according to claim 4 whereinthere is additionally provided a flip-flop circuit having a firstcondition and a second condition of stability and connected to saidanode of said normally conducting device to be driven from said firstcondition to said second condition by the suppressed conductingcondition of said normally conducting device, a teletypewriter selectmagnet, and a normally responsive electron device operativelyintreconnecting said code receiving circuit and said select magnet, saidnormally responsive electron device connected to said flip-flop circuitand caused to be rendered unresponsive only by said second condition ofsaid flip-flop circuit.

6. In a telegraph station selector system, a code signal receivingcircuit, a decoding circuit in said receiving circuit having a pluralityof output terminals and adapted to sequentially apply pulses to one ofsaid terminals in response to the reception of correspondingpredetermined signal by said receiving circuit, a self-restoringflip-flop circuit operable from a first condition to a second conditionto time an interval, means interconnecting a particular one of saiddecoding circuit terminals and said self-restoring flip-flop circuit fordriving said self-restoring flip-flop circuit from said first conditionto said second condition when a code signal corresponding to saidparticular one terminal is received, a second fiip-fiop circuit in afirst of two conducting conditions having a control means connectingsaid self-restoring flip-flop circuit and to another one of saiddecoding circuit terminals corresponding to another predetermined codesignal to cause a change in said second flip-flop circuit to the secondof said two conducting conditions in response to the sequentialhappenings of the operation to said second condition of saidself-restoring flip-flop circuit and the pulsing of said anotherterminal of said decoding circuit.

7. A system in accordance with claim 6 in which said control meansconnecting said second flip-flop circuit to said self-restoring fiipfiopcircuit comprises locking means to preclude the restoration of saidself-restoring flip-flop circuit to its first condition when said secondflip-flop circuit is in its second conducting condition.

8. A system in accordance with claim 7 wherein there is additionallyprovided a restoring control means connecting still another terminal ofsaid decoding circuit to said second flip-flop circuit to causerestoration of said second flip-flop circuit to its first conductingcondition and to render ineffective said locking control means when saidstill another terminal has a pulse applied there- 26 to corresponding tostill another predetermined code signal, and a reset means connectingsaid second flip-flop circuit to said self-restoring flip-flop circuitto cause restoration of said self-restoring flip-flop circuit to itsfirst condition in response to restoration of said second flipfiopcircuit to its first condition.

9. In a telegraph code signal generating circuit, a selfrestoringelectron discharge timing circuit operable from a first condition to asecond condition to time an interval, a free-running multivibratorcircuit operable while said timing circuit is in its second condition, anormally conducting device including a control grid, a pulse delaycircuit, a first control means connecting said self-restoring electrondischarge timing circuit to said control grid of said normallyconducting device including said pulse delay circuit to suppress currentflow in said normally conducting device at a time coinciding with apredetermined point in the operative cycle of said free-runningmultivibrator, a normally nonconducting device including a control gridand a second control means connecting said normally conducting device tosaid grid of said normally nonconducting device to cause conduction insaid normally nonconducting device when said normally conducting deviceis rendered nonconducting.

10. A telegraph code signal generating circuit in accordance with claim9 in which there is provided a transmission channel, a line relayconnected to said channel adapted to impress marking or spacingconditions thereon, said relay connected to said normally nonconductingdevice and operable to its spacing condition in response to a conductingcondition in said normally nonconducting device, a monostable flip-floptiming circuit operable from a first condition to a second condition totime an interval comprising a predetermined number of cycles of saidfreerunning multivibrator, said monostable flip-flop circuit connectedto said self-restoring timing circuit and operable to its secondcondition in response to the operation of said timing circuit to itssecond condition, a third control means connecting said multivibratorcircuit to said monostable flip-flop circuit to pulse said monostableflip flop circuit and control the restoration thereof to its firstcondition, a fourth control means connecting said monostable flip-flopcircuit to said normally nonconducting device to cause said normallynonconducting device to momentarily return to the nonconductingcondition thereof when said monostable flip-flop circuit restores to itsfirst condition, and a fifth control means connecting saidself-restoring electron device timing circuit to said normallyconducting tube to cause conduction in said normally conducting tubewhen said self-restoring timing circuit times said interval and restoresto its first condition.

11. A telegraph code signal generating circuit in accordance with claim10 wherein said first control means is adapted to suppress current flowin said normally conducting device at the end of the first one-halfoperative cycles of said free-running multivibrator and said monostableflip-flop timing circuit is adapted to operate its second condition totime an interval slightly longer than 3 /2 operative cycles of saidfree-running multivibrator.

References Cited in the file of this patent UNITED STATES PATENTS2,580,192 Potter Dec. 25, 1951 2,589,465 Weiner Mar. 18, 1952 2,774,868Havens Dec. 18, 1956 2,827,574 Schneider Mar. 18, 1958

